This invention is directed at a method for removing unsaturated hydrocarbon compounds from a process reducing gas stream utilized for metals passivation. More specifically, the present invention is directed at a method for removing olefins from a refinery process reducing gas stream utilized for passivating metals contaminated cracking catalyst.
In the catalytic cracking of hydrocarbon feedstocks, particularly heavy feedstocks, nickel, vanadium and/or iron present in the feedstocks become deposited on the cracking catalyst promoting excessive hydrogen and coke makes. These metal contaminants are not removed by conventional catalyst regeneration operations, which convert coke deposits on the catalyst to CO and CO.sub.2.
As used hereinafter, the term "passivation" is defined as a method for decreasing the detrimental catalytic effects of metal contaminants such as nickel, vanadium and/or iron which become deposited on the cracking catalyst. Several patents disclose the use of a reducing atmosphere to passivate cracking catalyst. U.S. Pat. Nos. 4,280,895 and 4,280,896 disclose that cracking catalyst can be passivated by passing the catalyst through a passivation zone having a reducing atmosphere maintained at an elevated temperature for a period of time ranging from 30 seconds to 30 minutes, typically from about 2 to 5 minutes. These patents disclose that process gas streams containing H.sub.2 and/or CO, such as cat cracker tail gas, catalytic reformer off-gas, spent hydrogen streams from catalytic hydroprocessing, synthesis gas and flue gases can be utilized.
U.S. Pat. Nos. 4,298,459 and 4,280,898 describe processes for cracking a metals-containing feedstock where the used cracking catalyst is subjected to alternate exposures of up to 30 minutes of an oxidizing zone and a reducing zone maintained at an elevated temperature to reduce the hydrogen and coke makes.
U.S. Pat. No. 4,268,416 also describes a method for passivating cracking catalyst in which metal contaminated cracking catalyst is contacted with a reducing gas at elevated temperatures to passivate the catalyst.
U.S. Pat. Nos. 4,361,496; and 4,364,848; and 4,382,015 describe metals passivation processes in which the metals contaminated catalyst is contacted with hydrogen and with a C.sub.1 -C.sub.3 hydrocarbon at an elevated temperature to reduce the metal contaminants and to selectively coat the contaminant sites with a layer of carbon.
European Patent Publication No. 52,356 also discloses that metal contaminants can be passivated utilizing a reducing atmosphere at an elevated temperature. This publication discloses that the reducing gas source can include regenerator off gases or light gases from the catalytic cracker.
International Patent Application No. WO82/04063 discloses the contacting of a regenerated catalyst with a reducing gas at elevated temperature to reduce oxidized nickel deposits on the cracking catalyst.
It also has been known to remove olefins, such as ethylene from hydrocarbon gas streams. In J.C.S. Chem. Comm. (1974) pages 584-585, Huang and Mainwaring disclose that ethylene formed a stable complex with copper (I) in a Type Y zeolite. In the Journal of Physical Chemistry, Vol. 70, No. 4, (1966) at pages 1126-1136, Carter et al also disclose that ethylene could be adsorbed on zeolites. U.S. Pat. No. 3,355,509 discloses the separation of olefins, particularly normal olefins, from other straight chain hydrocarbons utilizing cadmium or strontium substituted type X or type Y sieves. U.S. Pat. No. 3,150,942 discloses the use of molecular sieves to purify hydrogen.
Several U.S. patents disclose that copper containing compounds can be used to selectively remove an olefin from a gas stream. U.S. Pat. No. 3,437,713 discloses the use of cuprous halide complexes for selective olefinic separations including the selective separation of ethylene from a steam cracking stream and from ethane. U.S. Pat. No. 3,514,488 discloses a method for adsorption and desorption of olefins from saturated hydrocarbons utilizing copper complexes.
It has been found that the presence of unsaturated compounds, particularly olefinic compounds such as ethylene, in refinery gas streams contributes to excessive coke formation on the cracking catalyst. Excessive coke formation on the cracking catalyst is not desirable for several reasons. The presence of coke on cracking catalyst decreases the activity of the catalyst. Excess coke on the catalyst may also result in excess heat being liberated from the catalyst when the catalyst subsequently is regenerated. In some facilities where the regeneration zone capacity is limited by the air blower capacity, excess coke on the catalyst may require a reduction in the feed rate to the reaction zone.
Accordingly, it is desirable to provide a process which is capable of passivating metal contaminants on cracking catalyst utilizing available reducing gas sources without excess coke formation.
It also is desirable to provide a process for passivating cracking catalyst which is capable of utilizing reducing gas generated by the cracking zone.
It also is desirable to provide a process which is reliable and has a relatively low operating cost for selectively removing undesired compounds from reducing gas.
It also is desirable to provide a process which permits the separation and recovery of unsaturates from a process gas stream.
The subject invention is directed at a method for purifying commercially available reducing gas streams prior to their introduction into a passivation zone to passivate metal contaminants deposited on cracking catalyst. The present invention removes the unsaturated compounds, such as olefins, from the reducing gas prior to the addition of the reducing gas to the passivation zone. In a preferred embodiment the reducing gas is passed through a bed of zeolite adsorbent to remove the olefin. Metals exchanged zeolites are preferred, with a Cu(I)Y zeolite being especially preferred.